During the manufacture of paper, cardboard or similar, it is well known to introduce, into the pulp, retention agents whose function is to retain the maximum amount of fines and of fillers in the sheet. The beneficial effects which follow from the use of a retention agent are essentially:
increase in production and reduction in manufacturing costs: energy saving, more regular operation of machinery, higher yield of fibres, fines, fillers and anionic ennobling products, lower acidity in the circuit linked to a decrease in the use of aluminium sulphate and thus lessening of corrosion problems; PA1 improvement in quality: better formation and look through; improvement in the moisture content on the sheet, of the opacity, of the smoothness and of the absorbency and decrease in the porosity of the paper. PA1 13,240 kilograms of acrylamide at 30% in water; PA1 1,600 kilograms of a quaternary ammonium salt derived from methyl chloride and dimethylaminoethyl acrylate (DMAEA) at 75% in solution in water; PA1 50 kilograms of water and 100 kilos of adipic acid; PA1 and 0.129 kilogram of methylenebisacrylamide (MBA) (i.e. 25 ppm with respect to the active material) as branching agent.
It has long been proposed to add bentonite to the pulp, it being possible for the bentonite to be optionally added to other inorganic products, such as aluminium sulphates, or even synthetic polymers, especially polyethyleneimine (see, for example, documents DE-A-2,262,906 and U.S. Pat. No. 2,368,635).
In the document U.S. Pat. No. 3,052,595, it has been proposed to combine bentonite with a linear polyacrylamide. This process has not really been developed, because it has been competing with systems which are easier to implement while being just as effective. Additionally, even with current linear polyacrylamides, the retention power is still insufficient.
In the document EP-A-0,017,353, it has been proposed, for the retention of pulps containing only a small amount of fillers (at most 5% of fillers), to combine the bentonite with a weakly anionic, nonionic linear copolyacrylamide. This process has not really been developed, because these polymers have relatively little activity as regards retention, especially of pulps containing fillers, no doubt as a result of insufficient synergy between these copolymers and the bentonite which has little tendency to recoagulate.
In the document EP-A-0,235,893, it has been proposed to resort to essentially linear, or even partially crosslinked, cationic polyacrylamides having a molecular weight greater than one million, preferably thirty millions and more. In that way, an admittedly satisfactory retention effect is obtained, but still judged insufficient in the papermaking application, because, use of bentonite resulting in water treatment difficulties, users only select this system in the event of significant advantages.
In the article by TAPPI, published in Abstracts Bulletin of the Institute of Paper Science and Technology (vol. 62, No. 10, April 1992, page 1165), a mechanism of the supercoagulation of activated bentonite in the presence of a cationic copolyacrylamide was described, without the exact nature of it being specified. This process has the same disadvantages as previously.
The invention overcomes these disadvantages.